Combined cycle power plants are known in the art as an efficient means for converting fossil fuels to thermal, mechanical and/or electrical energy. Such systems are described in U.S. Pat. Nos. 4,932,204 dated Jun. 12, 1990, and U.S. Pat. No. 6,145,295 dated Nov. 14, 2000, both incorporated by reference herein.
It is also known in the art to cool selected combustion turbine parts with a cooling fluid. One such cooling fluid may be a portion of the compressed air produced by the compressor of the combustion turbine system. As firing temperatures have increased in order to improve the efficiency of combustion turbines, it has become desirable to use steam produced in the waste heat recovery steam generator of a combined cycle power plant as the cooling fluid for the gas turbine components, since steam has a higher heat transfer capacity than compressed air. Compressed air is used as the cooling fluid during the initial start-up of the plant until an adequate steam supply becomes available, at which time the cooling path is switched to steam flow. A portion of the steam produced in an intermediate pressure section of the waste heat recovery steam generator is typically used to provide this cooling flow. The temperature of the cooling steam will increase as it removes heat from the component being cooled. The steam is then directed to an intermediate pressure steam turbine where the heat energy is converted to mechanical energy. A second portion of the steam produced in the intermediate pressure section of the waste heat recovery steam generator is routed to the intermediate pressure steam turbine through a re-heater section of the waste heat recovery steam generator. There is an interaction between these two steam flows since they are both produced together in the intermediate pressure section of the waste heat recovery steam generator and they are delivered together to the intermediate pressure turbine.
As the power level increases during the start-up of a combined cycle power plant, the rate of flow of steam used for cooling the gas turbine components must be increased to accommodate the increasing combustion firing temperature. The pressure in the intermediate pressure section of the waste heat recovery steam generator is controlled as a function of power level to ensure that adequate steam pressure is available to drive the desired flow of cooling steam. A steam bypass valve is used to control the pressure in the steam drum by controlling the amount of steam flowing through the re-heater to the intermediate pressure turbine. However, changes in steam drum pressure demanded for the purpose of controlling the rate of flow of steam through the cooling circuit will affect the level of the water/steam interface in the steam drum. This can lead to unstable operation, especially at low power levels.
In order to provide the required cooling steam flow control while maintaining an adequate degree of steam drum pressure/level control, it is known in the art to provide a separate steam admission valve in the cooling steam flow circuit for controlling the flow rate of steam through the cooling circuit. Thus, steam drum pressure is controlled by selectively positioning the steam bypass valve, and steam cooling circuit flow rate is controlled by selectively and independently positioning the steam admission valve.
FIG. 3 illustrates the relationship between steam drum pressure and gas turbine power level for a prior art combined cycle power plant. Curve 100 is a cooling loop demand pressure curve representing the pressure required in the steam drum to produce the desired amount of cooling steam flow through the cooling circuit as a function of gas turbine power level. The absolute values assigned to points on this curve are plant specific, so curve 100 is provided without measuring units for illustration purposes. In order to ensure that this cooling steam flow demand can be satisfied, and in order to avoid steam system control problems, a bypass valve pressure control curve 102 is developed to control the steam drum pressure to a value that exceeds the demand value of curve 100 at all power levels. The pressure difference 104 between these curves 100 and 102 represents the amount of pressure loss that must be generated across the steam admission valve in order to produce the desired cooling steam flow rate.